Workfunction is the energy difference of the least bound electron of a solid and the electron's potential energy in vacuum outside the solid. The workfunction varies for different surfaces of solids. Photocathodes are devices that convert a fraction of the incident photons to emitted electrons. When a photocathode is struck by a photon, the absorbed energy causes electron emission due to the photoelectric effect. Low-workfunction photocathodes can be used in a variety of applications such as, without limitation, solar-cells, image-intensifier tubes, photomultipliers, field-emission displays, electron guns, free-electron lasers, and photosensors/detectors. Low-workfunction photocathodes are especially useful in night vision devices which magnify or intensify radiation in the visible and infrared spectrum. An exemplary low-workfunction photocathode is described in U.S. Pat. No. 3,821,778 to Kurtin which discloses an infrared low-workfunction photocathode for surveillance purposes constructed using a thin metal, Cs2O (cesium oxide) laminate.
In many photo-physical applications photoemissive materials are sought after that can turn a high fraction of the incident photons into emitted electrons, i.e. materials that have a high quantum-yield. Often, the quantum-yield of these materials depends heavily on the wavelength of incident photons. For many applications, ranging from electron-guns for synchrotrons and free-electron lasers to night vision devices, high quantum-yield photoemission using visible or infrared irradiation is desirable. In electron-guns of synchrotrons and free-electron lasers, emission in the visible range is advantageous for the improved control of the shape of the emitted electron bunch that is critical for time-resolved applications. In night-vision devices a very low flux of infrared photons has to be turned into emitted electrons with a high yield in order to obtain an image as sharp as possible. Therefore there is a quest for new and improved materials with optimized quantum-yield and low-workfunction.
One of the current best photoemissive materials is Cs2Te (cesium telluride). Cs2Te has been known since the 1950s and, conventionally, is widely used as a photoemissive material for photocathodes. Cs2Te is a high photo-yield compound. However, its workfunction is about 3 eV, which means only the photons in the ultraviolet spectrum cause electrons to be emitted. In electron-guns of synchrotrons and free-electron lasers, emission in the visible range is advantageous for the improved control of the shape of the emitted electron bunch that is critical for time-resolved applications. In addition to not being photoemissive in the visible spectrum, the surface of a photocathode made of Cs2Te oxidizes, reducing its photo-yield. Other photocathodes based on multi-alkali antimonide compounds, such as K2CsSb and (Cs)Na3KSb, have a relatively short operational lifetime especially when used in radio-frequency accelerating cavities. It is desirable to have a photoemissive material made of a material that has a high photo-yield like Cs2Te, but has a longer operational lifetime and a lower workfunction. Further, it is desirable to have a photoemissive material that functions in the visible light spectrum.